A crystal oscillation circuit is usually utilized as a clock signal source for an integrated circuit. The crystal oscillation circuit features high stability of frequency but difficulty in varying the frequency. Therefore, when a system requires a plurality of fundamental signal sources, the crystal oscillation circuit is often utilized as a high frequency clock signal source with high stability.
An RC oscillator comprises, in combination, capacitors, resistors, buffers and inverters. A plurality of inverters are connected in series, and both resistors and capacitors are connected in parallel to the inverters. An oscillation frequency f of the RC oscillator is determined by the values of these resistors and capacitors. When a threshold voltage V.sub.th of the transistors constituting the inverters and a source voltage V.sub.DD supplied to the transistors have the relation V.sub.th =1/2 V.sub.DD, then the oscillation frequency f is described as f=1/2.2 CR.
If the foregoing RC oscillator is incorporated into an integrated circuit (IC), the oscillation frequency f may vary due to variations of inverter characteristics, resistor values and capacitor values depending on manufacturing conditions. In order to solve the above-problems, a frequency adjustable RC oscillator has been developed to absorb the variations of each element.
Some examples of conventional frequency adjustable RC oscillators are disclosed in Japanese Patent Laid-Open Publications No. 60-260213, 63-116505 and 63-114304, respectively. These RC oscillators employ a plurality of capacitors (C) which determine the oscillation frequency f. These respective capacitors (C), except for prescribed capacitors, are connected to the inverters via switches which are controlled by decoders. The decoders perform a drive control of the switches in accordance with data inputted therein. In other words, the conventional frequency adjustable RC oscillator adjusts the oscillation frequency f by varying the value of the capacitor connected in parallel to the inverters, which is performed by driving the switch in accordance with data inputted to a control terminal of the decoder.
Hereinafter, the operation of an RC oscillator will be explained in detail. The RC oscillator initially generates a higher frequency f than the target frequency f.sub.t by only a prescribed number of capacitors C being connected to the inverters.
Next, an operator adjusts the frequency by measuring the frequency f and varying the control data supplied to the control terminal of the decoder in accordance with a deviation between the measured frequency f and the target frequency f.sub.t. The capacitor C which compensates for the deviation of the frequency f is connected via a switch in parallel to the inverters causing the oscillation frequency f to be adjusted to the target frequency f.sub.t. Then, the adjuster finalizes the frequency adjustment procedure by supplying the control data to the decoder.
The above described conventional technique, however, requires manual frequency adjustment by the adjuster for each of the respective integrated RC oscillation circuits. The adjuster must perform jobs, such as the measurement of the frequency, determination of input data to the decoder, and drive of the control terminal, independently, in accordance with each of integrated circuit devices. Therefore, the conventional frequency adjustment technique is time and cost consuming.